Russia runs a conscription system which means mercenaries are relatively more "volunteer" than their regular army.
The problem in comparing the PLA with Russia other than just that is that Russia is unable to fight an modern informationized war. In an informationized war, the valor of individual units is inconsequential compared to the ability to operate complex systems. Mercenaries do not have the ability to operate complex systems required in informationized war. That's why as nations have gotten more technologically advanced, mercenary usage has dropped dramatically amongst all the nations of the world. Mercenaries ultimately only fulfill the role of light infantry and all light infantry does in near-peer war is die.
I disagree about infantry being useless. A lot of future combat will be in mega-cities like Tokyo or Taipei which will require you to deploy troops with near suicidal disregard for their own lives in storming operations or just extreme amounts of aggression.
Drones won't be able to pick up all the slack any time soon due to jammers and the immaturity of robotics.